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Triplet excitations in graphene-based systems

In this article we investigate the excitations in a single graphene layer and in a single-walled carbon nanotube, i.e. the spectrum of magnetic excitations is calculated. In the absence of interactions in these systems there is a unique gap in the electron-hole continuum. We show that in the presence of Coulomb correlations new states, magnons, appear in this forbidden region. Coulomb interaction is examined in the context of Pariser-Parr-Pople (PPP) model which takes into account long range nature of interaction. The energy of new bound states depends on the strength of Coulomb forces. The calculations are performed for arbitrary electron-hole ($e-h$) momentum $\textbf{q}$ what allows to find the magnons dispersion law $\varepsilon(\textbf{q})$, effective mass $m^*$ and velocity $v_{gr}$. Finally, we determine the critical values of system parameters when this type of excitations can exist.